No genetic differentiation of giant clam (Tridacna gigas) populations in the Great Barrier Reef,Australia

Six Tridacna gigas populations were sampled in 1990 from locations throughout the central and northern Great Barrier Reef (GBR). Despite separations in excess of 1000 km, mean Nei's unbiased genetic distances among the populations was 0.0007. The complete lack of spatial variation observed among populations did not results from lack of genetic variability. Genetic variation within populations was high, with mean heterozygosities from 0.221 to 0.250. Gene frequencies were consistent with expectations under conditions of Hardy-Weinberg equilibrium. These data suggest panmixis, or random mating, throughout the highly connected reef system of the central and northern GBR. The large gene exchange among the giant clam populations has important implications for conservation management of one of the few large populations of T. gigas in the world. Small local effects are likely to be overcome in time by inputs from other sources. However, large genetic perturbations, particularly from up-current sources, may spread rapidly through the population.Contribution No. 561 from the Australian Institute of Marine Science     

Genetic structure of crown-of-thorns starfish (Acanthaster planci) in Australia

Significant genetic structuring is reported for crown-of-thorns starfish Acanthaster planci (L.) populations within Australia, sampled in 1987 and 1989, with Eastern Australian and Western Australian populations forming different groups. Eastern Australian populations sampled included the three most southerly, on isolated reefs of Elizabeth and Middleton Reefs, and Lord Howe Island. The Lord Howe Island population formed a discrete outlier from all other Australian populations, with average genetic distances (Nei's unbiased D) from other populations of 0.05 compared with 0.01 among all other populations. This is greater than the levels of genetic differentiation described for A. planci from sites at the extremes of the Pacific. The genetic distinction of Western Australia and Lord Howe Island is considered to reflect restricted dispersal as a result of current patterns. The Western Australian populations also showed less genetic diversity than the Great Barrier Reef populations, and this might reflect a founder effect or recent bottleneck in the Western Australian populations.Contribution No. 554 from the Australian Institute of Marine Science     

Genetic structure of outbreaking and non-outbreaking crown-of-thorns starfish (Acanthaster planci) populations on the Great Barrier Reef

The study used population genetic data to test whether outbreaks of crown-of-thorns starfish (Acanthaster planci) on the Great Barrier Reef (GBR) are largely derived from a single outbreak or are independent events. The consequences of different modes of outbreak for population differentiation and gene flow were predicted and then compared with those estimated from a set of 8 outbreaking and a set of 5 non-outbreaking populations sampled in 1986 and 1987. The level of inter-population variation observed among outbreaking populations was less than that among non-outbreaking populations and gene flow was greater among outbreaking than among non-outbreaking populations. Greater population differentiation among non-outbreaking than outbreaking populations was not consistent with the hypothesis that outbreaks were independent events, but was consistent with a number of outbreaks being secondary. Estimates which took account of a number of aspects of sampling error demonstrated significant levels of genetic subdivision among non-outbreaking populations but not among outbreaking populations. The lack of significant genetic subdivision of outbreaking populations, given significant levels among non-outbreaking populations, was also inconsistent with outbreaks being indenpendent events, but was consistent with outbreaks being largely secondary. The allozyme data were insufficient to identify clearly individual populations that might have been the result of an independent outbreak. It is concluded that the majority of outbreaks are probably secondary derivatives from a single primary outbreak occurring in the northern part of the Central Section of the GBR, although the possibility that a small number of populations might outbreak independently of the main set cannot be excluded.Contribution No. 547 from the Australian Institute of Marine Science     

Spatial heterogeneity of phytoplankton populations in estuarine surface microlayers

Phytoplankton populations are concentrated in surface microlayer waters of the York River and Mobjack Bay, Virginia, USA. The degree of vertical heterogeneity was variable and seemed most dependent on weather conditions (vertical mixing) and total phytoplankton cell densities. Vertical stratification also exhibited some taxocoenotic dependency. Structurally, the phytoplankton communities of the surface microlayer were less diverse than subsurface communities. In general, values of informational diversity (H'), evenness (J), and richness (S and r) are lowest in the surface microlayer and increase with depth (to 3.0 m). Mechanisms which structure and maintain vertical heterogeneity are discussed.This work was supported in part by funds from the National Science Foundation Research Applied to National Needs Program grants to the Virginia Institute of Marine Science and the Chesapeake Research Consortium.Contribution No. 791 of the Virginia Institute of Marine Science, and Contribution No. 62 of the South Carolina Marine Resources Center.     

Comparison of larval duration and pre- and post-settlement growth in two species of damselfish,Chromis atripectoralis andPomacentrus coelestis (Pisces: Pomacentridae), from the Great Barrier Reef

Light traps were used to capture larval fishes, immediately before settlement, at two localities 500 km apart on the Great Barrier Reef (GBR) in December, 1987. Samples from Lizard Island, in the northern GBR, and Davies Reef, in the central GBR, were dominated by two species of damselfish:Chromis atripectralis andPomacentrus coelestis. Analysis of otoliths revealed significant differences in both size and age at settlement between the two localities forP. coelestis, but not forC. atripectoralis. Growth rates determined for pre- and post-settlementP. coelestis suggested a sigmoidal growth trajectory through the larval life, with growth slowing as fishes approached the time of settlement. Post-settlement growth rates were faster than growth prior to settlement in both species. Growth in both species was, however, similar between localities. The relationship between fish size and otolith size was complex, varying both between pre- and post-settlement fishes, and among localities. This emphasizes the need to validate the relationship between fish size and otolith size before otoliths may be used to back-calculate individual growth trajectories.Contribution No. 500 from the Australian Institute of Marine Science     

Genetic differentiation among populations of a broadcast spawning soft coral, Sinularia flexibilis, on the Great Barrier Reef

The genetic structure of 12 reef populations of the soft coral Sinularia flexibilis (Octocorallia, Alcyoniidae) was studied along the Great Barrier Reef (GBR) at a maximum separation of 1,300 km to investigate the relative importance of sexual and asexual reproduction, genetic differentiation and gene flow among these populations. S. flexibilis is a widely distributed Indo-Pacific species and a gamete broadcaster that can form large aggregations of colonies on near-shore reefs of the GBR. Up to 60 individuals per reef were collected at a minimum sampling scale of 5 m at two sites per reef, from December 1998 to February 2000. Electrophoretic analyses of nine polymorphic allozymes indicated that genotypic frequencies in most populations and loci did not differ significantly from those expected from Hardy–Weinberg predictions. Analysis of multi-locus genotypes indicated a high number of unique genotypes (N _go) relative to the number of individuals sampled (N) in each reef population (range of 0.69–0.95). The maximum number of individuals likely to have been produced sexually (N*) was similar to the number of individuals sampled (i.e. N*:N ˜ 1), suggesting that even repeated genotypes may have been produced sexually. These results demonstrated a dominant role of sexual reproduction in these populations at the scale sampled. Significant genetic differentiation between some populations indicated that gene flow is restricted between some reefs (F _ST=0.026, 95% CI= 0.011 − 0.045) and even between sites within reefs (F _ST=0.041, 95% CI=0.027 − 0.055). Nevertheless, there was no relationship between geographic separation and genetic differentiation. Analyses comparing groups of populations showed no significant differentiation on a north-south gradient in the GBR. The pattern in the number of significant differences in gene frequencies in pairwise population comparisons, however, suggested that gene flow may be more restricted among inner-shelf reef populations near to the coast than among mid/outer-shelf populations further from the coast. Received: 10 July 2000 / Accepted: 5 October 2000     

Biodeposition by salt-marsh invertebrates

The importance of salt-marsh invertebrate biodeposits in biogeochemical cycles is discussed. Yearly biodeposition rates of nitrogen, phosphorus, potassium, magnesium, maganese, copper, zinc, and molybdenum are reported for 5 invertebrate species. The 5 species (Littorina irrorata, Arcatula demissa, Polymesoda caroliniana, Uca pugnax and U. pugilator) deposit 1709 g dry weight m² year^-1, of which 455 g m² year^-1 is organic material. Calculations suggest that the invertebrates are capable of processing 53% of the marsh's yearly production, but the actual amount utilized is probably substantially less than this.Contribution No. 716 from the Virginia Institute of Marine Science, and Contribution No. 295 from the University of Georgia Marine Institute.     

Salinity and substratum effects on condition index of the bivalve Rangia cuneata

The condition index (CI) of the clam Rangia cuneata was measured monthly at 8 stations in 2 substrate types and 3 salinity regimes in the James River estuary, Virginia, USA. CI was higher in clams from sand bottoms than from mud, and decreased in both substrata in fresher water. Values were lowest in early spring and at a maximum in early autumn, with a pronounced autumn peak in the sand substratum. This pattern coincided with the annual salinity cycle. A field experiment showed that some factor associated with the water overlying a sand or mud substratum, rather than the nature of the substratum itself, was important in determining CI. The hypothesis is presented that this factor may be the higher suspended solids concentrations measured immediately over mud bottoms.Virginia Institute of Marine Science Contribution No. 782.     

Latitudinal variation in linear growth and other shell characteristics of Macoma balthica

Shells of the bivalve Macoma balthica (L.) were studied from locations throughout its geographic range on both sides of the North Atlantic. These observations were supplemented by records from the literature. North-south clines were detected in shell coloration, relative shell dimensions and shell growth parameters. Moreover, shells from the two sides of the Atlantic were so different (particularly in the south) that these populations can hardly be regarded as conspecific. It is, therefore, argued that latitudinal changes should be studied separately for the American and European populations. In America, growth rates increased continuously to the south, whereas in Europe maximal growth was observed at more intermediate latitudes.Virginia Institute of Marine Science Contribution No. 1251     

Temperature-time relationships for survival of embryos and larvae of Mulinia lateralis (Mollusca: Bivalvia)

The influence of temperature over 8 time intervals on survival of cleavage stages, trochophore larvae, and straighthinge veliger larvae of Mulinia lateralis (Say) was investigated using a thermal gradient apparatus. There was a direct relationship of mortality with increased period of exposure. As the clams aged, temperature tolerance increased, with cleavage stages being most sensitive to higher temperatures and straight-hinge larvae least sensitive. Multiple-regression equations were developed to allow prediction of percentage mortalities under different conditions of temperature and time exposure. Entrainment of M. lateralis embryos and larvae in the cooling-water systems of steam-electric power plants should be as short as possible to keep mortality toa minimum.Contribution No. 550 of the Natural Resources Institute, University of Maryland, and Contribution No. 531 of the Virginia Institute of Marine Science.     

Asexual reproduction as a means of population maintenance in the coral reef asteroid Linckia multifora on Guam

The asteroid Linckia multifora (Lamarck) reproduces by autotomy of arms. Seven phases are recognized in its asexual reproductive cycle (autotomized arms; comets; counter-comets; post counter-comet I, II and III; and disc-parents). Individuals of the comet phase represent the highest percentage for population samples collected throughout the year from several sites on Guam, indicating a continuous asexual reproduction through which populations of L. multifora are being maintained.Supported by National Science Foundation Grant GA-39948 to the University of Guam Marine Laboratory; Contribution No. 108, University of Guam Marine Laboratory. Based on a thesis submitted in partial fulfillment of the requirements for the Degree of Master of Science at the University of Guam, USA.     

Complex patterns of population structure and recruitment of <Emphasis Type="Italic">Plectropomus leopardus</Emphasis> (Pisces: Epinephelidae) in the Indo-West Pacific: implications for fisheries management

Here the population genetic structure of an ecologically and economically important coral reef fish, the coral trout Plectropomus leopardus, is investigated in the context of contemporary and historical events. Coral trout were sampled from four regions (six locations) and partial mtDNA D-loop sequences identified six populations (Fst = 0.89209, P < 0.0001): Scott Reef and the Abrolhos Islands in west Australia; the Great Barrier Reef (GBR), represented by northern and southern GBR samples; New Caledonia and Taiwan, with Taiwan containing two genetic lineages. Furthermore, this study identified source and sink populations within and among regions. Specifically, the northern population in west Australia (Scott Reef) was identified, as the source for replenishment of the Abrolhos population, whilst New Caledonia was a source for recruitment to the GBR. Based on these insights from a single mtDNA marker, this study will facilitate the development of rational management plans for the conservation of P. leopardus populations and therefore mitigate the risk of population declines from anthropogenic influences.     

Resident mysids: community structure,abundance and small-scale distributions in a coral reef lagoon

Seasonal and diel variations in community structure and abundance of coral-reef lagoon mysids were examined at Davies Reef in the central region of the Great Barrier Reef (GBR) between June 1980 and May 1981. Twenty-five mysid species belonging to three subfamilies of the family Mysidae were captured during the study, with six new records for the GBR. The epibenthic mysid community differed from that in the overlying water, was faunistically uniform, but formed characteristic seasonal and diel groupings. The dominant epibenthic species were Erythrops sp., Anisomysis pelewensis, Doxomysis littoralis, A. laticauda, Prionomysis stenolepis, A. lamellicauda, and A. australis, five of which formed schools. Total mysid abundances ranged between 110 and 790 m^-3 with peak abundance in October. Schooling species occurred at local densities of up to 500 000 m^-3. Mysids were absent from shallow and midwater depths during the day, but were distributed throughout all depths at night with peak abundances in mid-water and deep layers. The dominant species in the water column at night were Pseudanchialina inermis, A. laticauda and Gastrosaccus indicus, in descending order of abundance. Lagoonal mysids contribute little to the food of sessile reef planktivores, as all but three species remain concentrated near or on the lagoon floor both day and night. The contribution of resident lagoon mysids to reef trophodynamics is probably through remineralization of lagoon detritus. Given the vast reef areas comprised of sandy lagoons, the large populations and relatively large size of lagoon mysids, this trophodynamic role may be of considerable importance.A.I.M.S. Contribution No. 477     

Heritability of juvenile growth for the hard clamMercenaria mercenaria

The heritability of juvenile growth in the hard clamMercenaria mercenaria (L.) sampled from Wellfleet Harbor, USA, in 1987, was analyzed in two experiments using standard quantitative genetic methods. In the first experiment, culture density was not controlled and was found to have a significant impact upon the non-additive genetic component of growth, possibly through genotype-environment correlation. Variable clam density appeared to have little impact on the additive genetic variance component. Estimates of the narrow sense heritability, calculated from the additive genetic component, ranged from 0.72 (±0.32) to 0.91 (±0.17). Culture density was controlled in the second experiment. The narrow sense heritability estimate from this experiment was 0.37 (±0.13). These estimates suggest that selection, either natural or artificial could bring about rapid change in the mean phenotype(s) of juvenile growth characteristics in hard clams.Contribution no. 793 from the Belle W. Baruch Institute for Marine Biology and Coastal Research     

Sexual and asexual production of planulae in reef corals

Electrophoresis was used to provide genetic evidence of the mode of production of brooded planulae in each of four species of scleractinian coral collected from the central region of the Great Barrier Reef during September, October and November 1984. Comparisons were made of the multi-locus genotypes of planulae and their broodparents for two ahermatypic (non zooxanthellate) species,Tubastraea diaphana andT. coccinea and two hermatypic (zooxanthellate) species,Acropora palifera andSeriatopora hystrix. For both ahermatypic species, all planulae were found to be genetically identical to their broodparents, including 91 planulae which were heterozygous for at least one locus. These results are consistent with asexual (ameiotic) reproduction. In contrast, non-parental genotypes were detected in the majority of hermatypic broods, which is consistent with expectations for sexual reproduction with at least some outcrossing. These data confirmed that brooded planulae may be produced both sexually and asexually and countered the suggestion that electrophoretic studies of hermatypic corals may be weakened by the contaminating effect of the enzymes of their symbiotic xooxanthellae.Contribution No. 306 of the Australian Institute of Marine Science     

Synergistic effects of temperature,salinity and light on the hermatypic coral Montipora verrucosa

Temperature tolerance in the reef coral Montipora verrucosa (Lamarck) is affected by salinity and light. Low salinity reduces ability of the coral to survive shortterm exposure to elevated temperature. High natural light intensity aggravates damage sustained by corals at high temperature. In long-term growth experiments, high light intensity caused substantial loss of zooxanthellar pigment, higher mortality rates, reduced carbon fixation and lowered growth rate at both upper and lower sublethal temperatures Effects of light at optimal temperature were less dramatic. Interactions between physical environmental factors appear to be most important near the limits of tolerance for a given factor. Acclimation capability was indicated, and was influenced by both thermal history and pigmentation state of stressed corals.Contribution No. 543 of the Hawaii Institute of Marine Biology.     

Seasonal changes in the structure of an intertidal community

Quantitative (0.25 m²) samples of macrofaunal (>1.0 mm) invertebrates were taken in each season from one habitat of an intertidal sandbar in the North Inlet estuary near Georgetown, South Carolina, USA. During all seasons the community inhabiting the sample site was numerically dominated by two species of haustoriid amphipods (Acanthohaustorius millsi and Pseudohaustorius caroliniensis). Seasonal changes at the community level were clearly controlled by the population dynamics of the numerically dominant species, and qualitative information on life histories was important to the interpretation of analyses' results.This work was supported by the Environmental Technology Center of Martin Marietta Corporation and the Belle W. Baruch Institute for Marine Biology and Coastal Research. It is Contribution No. 138 of the Belle W. Baruch Institute for Marine Biology and Coastal research.     

Genetic structure of dictyoceratid sponge populations on the western Coral Sea reefs

Allozyme variation at six polymorphic loci was examined in foliose dictyoceratid sponges from isolated reefs in the western Coral Sea. Four major genetic groups corresponding to the species Phyllospongia lamellosa, P. alcicornis, Carterospongia flabellifera and Collospongia auris were examined. A further two rare morphotypes from individual reefs formed genetic outliers to the P. lamellosa group, and may represent further taxa related to P. lamellosa. Gene frequencies in individual reef populations were largely in Hardy-Weinberg equilibrium, suggesting that random mating occurred in local populations of all four common species. Genetic variability was high and observed heterozygosities within populations ranged from 0.13 to 0.40. All four taxa showed significant genetic differentiation among populations (F _ST=0.05 to 0.36). Genetic distances (Nei's D) among populations within species ranged from 0 to 0.723 and increased with increasing geographical separation. There was evidence that genetic differentiation between populations to the north and to the south of the southern limit of the South Equatorial Current (SEC) divergence was greater than expected on the basis of their geographical separation. The SEC divergence may form a partial barrier to gene flow among populations of these ecologically important sponges on the submerged Queensland Plateau. Levels of migration among populations of three of the species was less than those required to prevent divergence of the populations through genetic drift (Nm<1). Restricted migration among populations may provide a mechanism to explain the occurrence of highly divergent populations of dictyoceratid sponges whose specific identity is not clear, and may allow them additionally to develop partial reproduction isolation from other populations.     

Faunal variation on pelagic Sargassum

Pelagic Sargassum was collected in late summer, late winter, and early and late spring from inshore waters, the Gulf Stream and the Sargasso Sea of the Western North Atlantic Ocean. The noncolonial macrofauna was picked from the weed samples. The 34 samples contained 67 species and 11,234 individuals. The Shannon-Wiener index of diversity had a mean value of 2.419±0.177 (t.₀₅s _x ^– ) and a statistical range between 1.401 and 3.437 (t.₀₅ s). Mean diversity values were not significantly different among the various sampling series, and diversity did not vary with raft volume. High diversity values were related to an equitable distribution of species resulting from a stable environment and an area low in productivity. Species composition of the Sargassum organisms varied seasonally and geographically. Animals were more abundant in the spring than in the fall samples. Samples collected on a transect in the Gulf Stream and Sargasso Sea maintained a similar faunal composition.Contribution No. 351 from the Virginia Institute of Marine Science, Gloucester Point, Virginia, USA.From a thesis submitted to the faculty of the School of Marine Science, The College of William and Mary, in partial fulfillment of the requirements for the degree of Master of Arts in Marine Science.     

Effect of light quality on photosynthesis of the reef coral Montipora verrucosa

Pieces of the reef coral Montipora verrucosa (Lam.), collected from Kaneohe Bay, Oahu, Hawaii in 1982, were grown in four low-light treatments (11% sunlight): blue, green, red and the full spectrum of photosynthetically active radiation (PAR); and at high-intensity full PAR (90% sunlight). These acclimated corals were then tested for photosynthetic ability in blue, green, red, and white light. The photosynthetic parameters that were measured were; ligh-saturated photosynthetic rate, the initial slope of the photosynthesis/irradiance curve, the light intensity where these two lines crossed, and dark respiration. While acclimation intensity had a pronounced effect, the results also showed that the color of the acclimation treatment influenced the photosynthetic responses of the corals. The color of the light used in the measurements of photosynthesis had much less effect on the photosynthetic responses of the corals.Contribution No. 729 of the Hawaii Institute of Marine Biology